This invention relates to an organic electroluminescent element.
An organic electroluminescent element is an element formed using organic compounds as a luminescent material which luminesces in conjunction with electrical signals.
An organic electroluminescent element comprises an organic luminescent layer disposed between a pair of opposing electrodes. Luminescence is the phenomenon occurring when electrons are injected from one electrode and holes are injected from the other electrode to excite the luminescent material in the luminescent layer to a high energy potential, which induces the surplus energy to be discharged as light when the excited luminescent material returns to its original state.
In addition to the aforesaid basic structure, a hole injection layer is added to the electrode which injects holes, and an electron transporting layer is added to the electrode which injects electrons so as to improve the luminescence efficiency.
An example of an organic electroluminescent element is disclosed in U.S. Pat. No. 3,530,325, which uses monocrystalline anthracene as a luminescent material.
Japanese Laid-Open Patent Application No. SHO 59-194393 discloses the combination of a hole injection layer and an organic luminescent layer.
Japanese Laid-Open Patent No. SHO 63-295695 discloses the combination of an organic hole injection/transporting layer and an organic electron injection/transporting layer.
These multilayered electroluminescent elements are structured as multiple layers of an organic luminescent material, and a charge transporting organic material (charge transporting member), and electrodes, which move electrons and holes injected by the respective electrodes through the charge transporting material, and luminesce when the holes and electrons re-combine. Examples of useful organic luminescent materials include organic luminescing colorants such as 8-quinolinol-aluminum complex, and coumarin compounds. Examples of useful charge transporting materials include Nxe2x80x2-di(m-tolyl)N,Nxe2x80x2-diphenylbenzidine, amino compounds such as 1,1-bis[N,N-di(p-tolyl)aminophenyl]cyclohexane, 4-(N,N-diphenyl)aminobenzaldehyde-N,N-diphenylhydrazone compounds, and the like. Porphyrin compounds such as copper phthalocyanine have also been proposed.
Organic electroluminescent elements have high luminescence characteristics, but have not attained practicality due to their insufficient stability while luminescing and storage stability. The stability of the charge transporting layer is indicated as one problem relating to stability of the element during luminescence and to the storage stability of the element. The layers formed by the organic material of the electroluminescent element are very thin, to the order of several tens to several hundred nanometers, and the voltage applied per unit thickness is extremely high. Exothermic heat is produced by luminescence and current flow, which requires electrical, thermal, and chemical stability of the charge transporting layer.
The use of materials other than aluminum for the negative electrode to reduce the luminescence starting voltage of the organic electroluminescent element has been disclosed in Japanese Laid-Open Patent Application Nos. HEI 2-15595, 3-37994, 4-132191, and 5-121172. The use of metal oxide materials as the electron injection layer has also been reported in Appl. Phys. Lett. 70 (2) 152-154.
The use of metals other than aluminum produces disadvantages such as difficult layer forming conditions and easy oxidation. Even when metal oxides are used, layer formation is difficult inasmuch as the layer must be made extremely thin. Accordingly, it is difficult to produce an organic electroluminescent element which has a high luminescence intensity and maintains stable performance with repeated use.
In view of the aforesaid background, a feature of the invention is to provide an organic electroluminescent element which has a high luminescence intensity and maintains performance stability with repeated use.
The present invention relates an organic electroluminescent element comprising at least a positive electrode, an electron injection layer, and a negative electrode, wherein the electron injection layer is formed of an alkali metal organic salt, an alkaline earth metal organic salt, an alkali metal organometallic complex or an alkaline earth metal organometallic complex.
A hole injection/transporting layer may be interposed between the positive electrode and the luminescent layer. An electron transporting layer may be interposed between the luminescent layer and the electron injecting layer. The electron injection layer may be formed as a mixed layer formed of a mixture of an alkali metal organic salt, an alkaline earth metal organic salt, an alkali metal organometallic complex and/or an alkaline earth metal organometallic complex.